Various methods for converting an AC voltage into a DC voltage are generally known. A first method uses a diode bridge circuit and a smoothing capacitor. The diode bridge circuit performs full-wave rectification of an alternating current obtained from an AC power source. The smoothing capacitor smoothes a direct current obtained after full-wave rectification.
According to the first method, regardless of whether the AC voltage is positive or negative, a current always flows in a series circuit of two diodes in the diode bridge circuit. In the series circuit of two diodes, a power loss occurs that corresponds to a product of the current flowing in the respective diodes and a forward voltage of each diode.
A second method interposes a power factor improving converter (PFC) between the diode bridge circuit and the smoothing capacitor. The power factor improving converter controls the current flowing in the AC power source to be sinusoidal, and controls the current to be equal to a voltage phase of the AC power source. According to the second method, power loss also occurs, since the current flows in the series circuit of the two diodes during the full-wave rectification. In addition, the current flows alternately in a field effect transistor (FET) of the PFC and the diode. Consequently, a greater power loss occurs.
The power factor improving converter has to be set so that an output voltage is higher than an input voltage, since it is necessary to set a waveform of an input current to be a sine wave. However, a voltage required by a load is not necessarily a voltage that is higher than the input voltage. In that case, a step-down converter is connected to a rear stage of the power factor improving converter. Then, the voltage stepped up by the power factor improving converter is stepped down to a desired voltage. Even during the step-down of the voltage, the power loss continues to occur. An overall power conversion device is configured to have three stages of AC-DC conversion, DC-DC (step-up) conversion, DC-DC (step-down) conversion. Power conversion efficiency is represented by the product of conversion efficiency of these stages. For example, if the efficiency per one stage is assumed to be 0.95, the efficiency in the third stage is expressed by 0.95×0.95×0.95=0.86. That is, even if the conversion is performed so that the efficiency of an individual stage is 95%, the efficiency falls to 86% over three stages. As described above, even if conversion efficiency of an individual stage is good, the conversion efficiency in the case of multiple stages can be poor.
Recently, electronic devices are required to consume less power. At the same time, it is an essential requirement not to generate current harmonic noise so that the noise does not adversely affect the external environment. For this reason, there is a need for both improved conversion efficiency of the power conversion device supplying power to a load and a current harmonic suppression function.